Wireless telecommunications systems typically include various types of forward link channels that are used to transmit signals from base transceiver stations (BTSs) to mobile stations and various types of reverse link channels that are used to transmit signals from mobile stations to BTSs. The reverse link channels may include reverse traffic channels that are assigned to mobile stations for particular communications sessions, such as voice calls, and access channels that mobile stations are able to use to communicate with the system. For example, a mobile station may use an access channel to originate a call or to respond to a page.
In certain types of air interfaces, access channel resources are not allocated to particular mobile stations. Instead, mobile stations may contend with one another for usage of an access channel. To contend for an access channel, a mobile station may make an “access attempt,” for example, by transmitting a series of access probes. Each access probe may include the message that mobile station is trying to convey (e.g., a call origination message, page response, registration message, etc.). Within each access probe, the message may be preceded by a preamble. The preamble may be a predefined sequence of symbols that allows a BTS to recognize the beginning of the access probe and, thereby, receive the message contained in the access probe.
In accordance with IS-95 CDMA standards, the preamble is a sequence of zeroes (other standards may provide for other types of preambles), and the signals in the access probe are encoded by a “long” pseudonoise (PN) code that the mobile station generates using an access channel long code mask. The mobile station constructs the access channel long code mask based on parameters that relate to the BTS that is intended to receive the access probe. The known sequence of zeroes in the preamble allows the BTS to decode the signals in the access probe and, thereby, retrieve the message. In this regard, the preamble should be long enough so that the BTS has enough time to determine how to decode the signals in the access probe.
In IS-95 CDMA standards, the length of the preamble is defined by a size parameter, PAM_SZ. More particularly, the preamble has a length of 1+PAM_SZ frames, where each frame is a sequence of 96 “0” bits. PAM_SZ may take on integral values from 0 through 15, so that the preamble length may be between 1 and 16 frames. Typically, a wireless telecommunications system uses a fixed value for PAM_SZ, such as PAM_SZ=2.